One identity or more for telomeres?

Abstract

A major issue in telomere research is to understand how the integrity of chromosome ends is controlled. The fact that different types of nucleoprotein complexes have been described at the telomeres of different organisms raises the question of whether they have in common a structural identity that explains their role in chromosome protection. We will review here how telomeric nucleoprotein complexes are structured, comparing different organisms and trying to link these structures to telomere biology. It emerges that telomeres are formed by a complex and specific network of interactions between DNA, RNA, and proteins. The fact that these interactions and associated activities are reinforcing each other might help to guarantee the robustness of telomeric functions across the cell cycle and in the event of cellular perturbations. We will also discuss the recent notion that telomeres have evolved specific systems to overcome the DNA topological stress generated during their replication and transcription. This will lead to revisit the way we envisage the functioning of telomeric complexes since the regulation of topology is central to DNA stability, replication, recombination, and transcription as well as to chromosome higher-order organization.

Keywords: DNA topology; capping complexes; telomeres; telomeric chromatin organization.

Figure 1

Versatility of telomeric DNA structures. (A) G-quadruplex. (B) t-loop. (C) i-motif. (D) G-C hairpin end. G-rich and C-rich strands are represented in blue and green respectively.

Figure 2

TRF2 protein complexes with nucleic acids, a working model. TRF2 binds to DNA and modifies its topology. This intrinsic property of TRF2 is inhibited by the binding of TERRA (in red in the figure) synthesized by RNA polymerase II (represented in violet).

Figure 3

The diversity of protein capping complexes in different species. When solved, the 3D structures of proteins or domains are shown (for pdb entry numbers refer to Giraud-Panis et al., 2010a).

Figure 4

Cartoon representations of ss-DNA and ds-DNA binding domains. (A) Canonical OB-fold, with β-strand 1–5 and α-helix 1 (pdb entry 1QZG). (B) OB-fold from human Pot1 (1QZG) in complex with ss-DNA (dark gray), the additional secondary structures is shown in purple. (C) Canonical Myb-fold, with α-helix 1–3 (1W0U). (D) Myb domain from human TRF2 in complex with ds-DNA. (E) Double Myb domain from budding yeast Rap1 in complex with ds-DNA. (F) Alignment of TRF1 and TRF2 telobox sequences across different species allows definition of a consensus sequence and thus of a different signature between them (for details on the species used for the alignment, see Poulet et al., 2012).

Figure 5

Telomeric nuclear tethering. (A) S. cerevisiae. Schematic representation of Sir4–Esc1 anchoring and yKu-Mps3 anchoring pathways of yeast telomeres. (B) Humans. Several pathways are proposed to tether human telomeres to the nuclear matrix. The tethering of human telomeres was proposed to depend on TIN2L. Lamina, a component of the nuclear matrix, is linked to the nuclear membrane by LAP proteins and by a Sun/Kash type complex. CTCF participates in the nuclear localization of subtelomers via lamina. (C) S. pombe Bouquet. During meiosis, Bqt1 and Bqt2 proteins join Rap1 and Taz1 to Sad1, thus tethering telomeres to the nuclear envelope. The Sad1-Kms1 complex anchors the whole structure to microtubules.

Figure 6

TRF2 as a topological stress sensor. Due to topological constraints, during replication, opening of the double helix produces pre-catenanes behind the fork and positive supercoils ahead. TRF2 binding to these positive supercoils would allow the recruitment of enzymatic activities (Apollo for instance) that would help to relieve the topological stress.

Figure 7

One terminal problem, different solutions? Some telomeric features are universal but others are less conserved. Telomeric DNA can adopt diverse structures (5′ overhang, G4, and t-loop for examples) and can even lack repeats (Drosophila, HAATI telomeres). Failing specific sequences, heterochromatin can provide a backup system for protection. Although heterochromatic properties have been linked with peripheral localization in budding yeast, this telomere positioning is not a widespread feature. Conversely, transcription seems to be shared by all telomeres studied so far. Capping proteins are also major components of telomeres, particularly overhang binding proteins. Recent data suggest that topological issues may be of particular relevance at telomeres. Topological stress may constitute a conserved signaling pathway to recruit end capping proteins.